Dolly was able to be cloned successfully because the technique of "somatic cell transfer" was used. However, cloning cattle is becoming difficult and the success remains low (fewer than 10% survive to birth). The main reason the cattle don't survive is embryonic death. Cloning cattle is important because it can be used to study mammalian development.
Harris Lewin, professor in the UC Davis Department of Evolution and Ecology, conducted a study using RNA sequencing to understand gene expression in cloned cows during implantation to discover why there is a high rate of failed pregnancy for cloned cows. The study combined the French's expertise in cloning and reproductive biology with the U.S.'s expertise in functional genomics. This collaboration lead to understanding the mechanisms that account for embryonic losses at implantation. The study also provided insight on how implantation events drive the progression of pregnancy and shape the phenotypes of the cattle after they are born.
The researchers studied the tissue of cloned cow embryos at 18 and 34 days of development and the endometrial lining of the cows pregnant with the clones. They also studied cows that conceived by artificial insemination with non-cloned cows. The study resulted in the finding of multiple genes that are expressed abnormally. This could explain why there is a high rate of death for cloned embryos. The researchers found abnormalities in the expression of more than 5,000 genes on day 18. Results also suggest that that the surviving clones were able to successful implant in the uterus and form a placenta, indicating that the losses of clones is due to problems of critical development genes in the extra embryonic tissue. The study also revealed other factors that could be leading to the death of the clones including problems with hormone signaling between the pregnant cow and the developing embryo.
I found this article to be fascinating. I thought if scientists were able to clone a sheep it would only be a short jump to cloning cows too. I didn't think that cloning cows in the same way as Dolly would be so difficult. This article makes me ask the question if any animals can be cloned the same way or if each animal has to be cloned separately. This study could possibly provide further insight to cloning other mammals.